A steel sheet for vitreous enameling was conventionally produced by subjecting a capped steel or a rimmed steel to ingot casting, break down rolling, hot rolling, cold rolling, and then, open coil annealing for decarbonization and further denitrification annealing for lowering the contents of carbon and nitrogen to several tens of ppm or less. However, a steel sheet for vitreous enameling produced through these processes had the following shortcomings: the steel sheet was manufactured through the ingot casting and break down rolling processes; the annealing processes for decarbonization and denitrification were required; and, as a consequence, the cost of manufacturing was high.
In this background, technologies of producing a steel sheet for vitreous enameling by employing continuous casting were developed aiming at overcoming the shortcomings. At present, it is a common practice to produce a steel sheet for vitreous enameling by the continuous casting method for reducing the manufacturing cost. As an example of such technologies, Japanese Unexamined Patent Publication No. H07-166295 discloses a technology of producing a steel sheet for vitreous enameling by subjecting a high-oxygen steel to continuous casting. However, a steel sheet for vitreous enameling produced by the technology is inferior in enameling properties and is not applicable to deep-drawn products having complicated shapes.
The finding that an addition of Nb and V makes it possible to produce a steel sheet for vitreous enameling having good workability and enameling properties has been disclosed in Japanese Unexamined Patent Publication No. H1-275736. This is an epoch-making technology in which Nb and V are added as elements capable of maintaining a high oxygen content in a steel, thanks to their low deoxidation capacity, and create good workability by fixing C and N in the steel in the form of carbide and nitride. Besides this, although it is not related to the enameling properties and workability, Japanese Patent No. 2040437 regarding a steel sheet for vitreous enameling containing Nb and V, wherein the swelling likely to peculiarly take place during casting under special conditions is prevented by adding Sn, has been disclosed.
In addition, as a result of efforts to improve a steel sheet for vitreous enameling containing Nb and V and being excellent in fish scale resistance and deep drawability, the present inventors filed Japanese Patent Application No. 2000-390332. However, although a steel sheet according to the proposed technology secures a high and stable r-value, it is not sufficient to attain fish scale resistance as good as or better than that of a purely Al-free, high-oxygen steel simultaneously with a good r-value. It is known that, for suppressing fish scales of a steel sheet for vitreous enameling, it is effective to form voids in a steel sheet and trap hydrogen which has penetrated into the steel sheet in the voids during the baking of vitreous enamel. However, the mere formation of voids does not necessarily increase the capacity to trap hydrogen. The influence of a steel chemical composition on vitreous enameling properties has been pointed out in various technologies, and various technologies of prescribing a steel chemical composition especially for improving fish scale resistance have been disclosed.
It is publicly known that the addition of Nb and V makes it possible to produce a steel sheet for vitreous enameling having good workability and enameling properties, for instance, through above-mentioned Japanese Unexamined Patent Publication No. H1-275736 and Japanese Patent No. 2040437. While these technologies may be interpreted, from the viewpoint of fish scale resistance, as those proposing the formation of voids and the improvement of the hydrogen trapping capacity of the voids, it is hard to say that the optimum control from the viewpoint of the volume, shape and nature of the voids is employed in the technologies. As a result, the technologies are insufficient to improve fish scale resistance and the application thereof to practical use is hindered.